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Embryology Timeline
11
Alex Edwards
9.4.22
Embryology Timeline
PRE-IMPLANTATION
STAGE
Pre-Implantation: period from fertilization in fallopian tubes to implantation in the uterus
Week1 Development:
- fertilization of ovum by sperm to form zygote
- cells are going through mitosis within zygote
- 5th day - blastocyst forms
- 6-7th day - implantation into the endometrial lining
EMBRYOIC
STAGE
BEGINS
Embryonic Stage: all major organs and structures form within this period (carried out from the start of week 2 to the end of week 8)
Week 2 Development:
- blastocyst forms embryonic cell layers (germ layers)
- bilaminar embryonic disk is developed (epiblast and hypoblast)
- placenta forms from trophoblast layer and the endometrial lining
WEEK 3
- amount of HCG produced rapidly increases
- primitive streak is formed - from movement of cells from epiblast layer
- cephalic and caudal ends are formed
- trilaminar disk forms (ectoderm, mesoderm, endoderm)
- CNS starts to develop - neuroectoderm cells differentiate from ectoderm and neural plate, neural groove, and neural folds are formed
- neural crest cells disperse into mesenchyme
- stomodeum begins forming - ectoderm invaginates on ventral side until it encounters foregut
- formation of the buccopharyngeal and oropharyngeal membrane
WEEK 4
- neural tube forms (future spinal cord and brain)
- embryonic folding occurs - proliferation of the ectoderm
- inner tube of endoderm will form the future digestive tract
- primitive face starts to develop
- heart and other organs also start developing
Development of the Face
- buccopharyngeal/oropharyngeal membrane ruptures - making an open GI tract
- mandibular arch (1st branchial arch) and related tissues start to develop - mandibular processes appear below stomodeum
- tissue swelling forms from increased growth of mandibular arch to form maxillary process
- frontonasal process forms bulge of tissue at most cephalic end
- lens, otic, and nasal placodes are present
- development of nasal region and nose begins - tissue undergoes growth
- upper lip formation begins - joining of maxillary and 2 medial nasal processes
Development of the Neck
- 6 brachial arches begin to develop inferior to stomodeum
- brachial grooves are present
- tongue begins developing - formed by the first 4 branchial arches (tuberculum impar is first sign)
WEEK 5
- brain and face continue growing
- heart starts beating
- nervous system and circulatory system begin to differentiate
- mesoderm of mandibular arch start to form muscles of mastication
- depressions that will become nostrils are visible
- beginning of retinas form
- upper lip continues forming
- lateral lingual swellings merge to start to form anterior 2/3rds of tongue
WEEK 6
- small swellings of future ears develop
- eyes become apparent
- upper lip is almost done forming - from the 2 medial nasal processes and maxillary process
- maxillary process gives rise to 2 palatal shelves "lateral palatine processes"
- nasal cavity continues forming with palate
- trunk and neck begin to straighten
WEEK 7
- eyelids form
- primitive nervous system forming
- breathing tubes from throat to lungs are being formed
- odontogenesis of the primary dentition begins - initiation stage
- intermaxillary segment is formed
- fusion of processes to form upper lip is completed
- mandibular and maxillary process join creating labial commissure
- last four brachial grooves are obliterated, and pharyngeal pouches develop
END OF EMBRYONIC STAGE
Week 8:
- base and body of tongue fuse - copula merges with anterior swellings
- tongue moves anteriorly and inferiorly to allow palatal shelves to "flip"
- lingual papillae being to appear
- bud stage of tooth development
- facial features become more pronounced
- eyes begin to develop pigment
- fetal heartbeat can be monitored
Trial
FETAL
STAGE
BEGINS
Fetal Period: final development and maturation of all the tissues (beginning of 9th week to birth)
Week 9:
- face is broad - eyes widely separated, eyelids fused, and ears low set
- red blood cells beginning to form in liver
- cap stage of tooth development - formation of tooth germ as enamel organ forms
- fusion of nasal septum and fusion of palate begin
WEEK 10
- more developed profile
- head is getting bigger due to rapid development of the brain
- eyes close completely
- mandibular muscle masses have become four muscles of mastication
- lateral pterygoid muscle cells extend horizontally
- final palate continues forming
- filiform lingual papillae develop
- continuation of cap stage of tooth development
WEEK 11
- beginning of the bell stage of tooth development - differentiation of enamel organ into bell shape
- palate continues to fuse with 3 palatal processes and nasal septum
- taste buds begin to develop
- filiform papillae finish developing
WEEK 12
- fusion of 3 processes of palate is complete
- separation of oral and nasal cavities is complete
- mesenchyme from branchial arches 1 and 2 migrate to form soft palate muscles
- fusion of vertical nasal septum and palate is complete
- taste buds continue to develop
- end of bell stage of tooth development
- nerves start sending out simple signals for reflex
REFERENCES
Fehrenbach, M. J. (2020). Illustrated Dental
Embryology, histology, and Anatomy Margaret J. Fehrenbach, Tracy Popowics. Elsevier.
Mayo Clinic. (2022, June 3). Fetal
development: What happens during the 1st trimester? Mayo Clinic. Retrieved September 4, 2022, from https://www.mayoclinic.org/healthy-lifestyle/pregnancy-week-by-week/in-depth/prenatal-care/art-20045302
VinMec International Hospital. (2019). 11
week fetal development. vinmec.com. Retrieved September 4, 2022, from https://www.vinmec.com/en/news/health-news/obstetrics-gynecology-and-assisted-reproductive-technologies-art/11-week-fetal-development/